Crude oil in commercial quantities is generally found in the pore space in sedimentary rocks; less than one percent of the world's oil has been found in fractures in igneous or metamorphic rocks, about fifty-nine percent has been found in pores between the mineral grains of sandstones, and about forty percent in the void space present in dolomites or limestones (carbonates).
The two most important characteristics of a reservoir rock are its porosity and its permeability. Porosity is defined as the ratio of the volume of pore space to the total bulk volume of the material expressed in percent. Permeability is the capacity of the rock to transmit fluids through the interconnected pore spaces of a rock; the customary unit of measurement is the millidarcy. Although there often is an apparent close relationship between porosity and permeability, because a highly porous rock may be highly permeable, there is no real relationship between the two; a rock with a high percentage of porosity may be very impermeable because of a lack of communication between the individual pores or because of capillary size of the pore space.
After a borehole has penetrated the possibly productive formations, these formations must be tested to determine if expensive completion procedures should be used. The first evaluation is usually made by well-logging methods, in which the logging tool is lowered past the formations while the response signals are relayed to operators on the surface. Often these tools make use of the differences in electrical conductivities of rocks, water, and petroleum to detect possible oil or gas accumulations. Other logging tools depend on difference in absorption of atomic particles. Well-logging tools identify the productive formations which are further verified by a production test.
If the preliminary tests show that one or more of the formations in the borehole will be commercially productive, the well must be prepared for the production of the oil or gas. First, a large outside pipe, or casing, slightly smaller in diameter than the drill hole, is inserted into the full depth of the well. A cement slurry is forced between the outside of the casing and the inside surface of the drill hole. When set, this cement forms a seal so that fluids cannot pass from one portion of the well to the other through the borehole. The casing is usually about nine inches (23 centimeters) in diameter. It creates a permanent well through which the productive formations may be reached. After the casing is in place, a production string of smaller tubing is extended from the surface to the productive formation with a packing device to seal the productive interval from the rest of the well. If multiple productive formations are found, as many as four production strings of tubing may be hung in the same cased well. If a pump is needed to lift oil to the surface, it is placed on the bottom of the production string.
Since the casing is sealed against the productive formation, openings must be made to allow the oil or gas to enter the well. A down-hole perforator uses an explosive to shoot holes through the casing and cement into the formation. The perforator tool is lowered through the tubing on a wire line. When it is in the correct position, the charges are fired electrically from the surface. Such perforating will be sufficient if the formation is quite productive. If not, an inert fluid may be injected into the formation at pressures high enough to fracture the rock around the well and thus open more flow passages for the petroleum. In early times, nitroglycerin was exploded in the well bore for the same purpose.
The permeability of an earth formation containing valuable resources is a parameter of major significance to the economic production of the resource. These resources are generally located by borehole logging which measures the resistivity and porosity of the formation in the vicinity. Such measurements enable porous zones to be identified and their water saturation (percentage of pore space occupied by water) to be estimated. A value of water saturation significantly less than unity is taken as being indicative of the presence of hydrocarbons, and may also be used to estimate their quantity. However, this information alone is not necessarily adequate for a decision on whether the hydrocarbons are economically producible. The pore spaces containing the hydrocarbons may be isolated or may be only slightly interconnected, in which case the hydrocarbons will be unable to flow through the formation to the borehole. The ease with which the fluids can flow through the formation (also known as permeability), should preferably exceed some threshold value to assure the economic feasibility of turning the borehole into a producing well. The threshold value may vary depending on such characteristics, such as viscosity in the case of oil. For example, a highly viscous oil will not flow easily in low permeability conditions and if water injection is to be used to promote production, there may be a risk of premature water breakthrough at the producing well.
The permeability of a formation is not necessarily isotropic. In particular, the permeability for fluid flow in a generally horizontal direction may be different from (and typically greater than) the permeability value in a generally vertical direction. This may arise, for example, from the effects of interfaces between adjacent layers making up a formation, or from anisotropic orientation of formation particles such as sand grains. Where there is a strong degree of permeability and anisotropy, it is important to distinguish the presence and degree of the anisotropy, to avoid using a value dominated by the permeability in only one direction as a misleading indication of the permeability in all directions.
Present techniques for evaluating the vertical permeability of a formation are somewhat limited. One tool that has gained commercial acceptance provides for repeat formation testing (RFT) and is described in U.S. Pat. Nos. 3,780,575 and 3,952,588. This tool includes the capability for repeatedly taking two successive samples at different flow rates from a formation via a probe inserted into a borehole wall. The fluid pressure is monitored and recorded throughout the sample extraction period and for a period of time thereafter. Analysis of the pressure variations with time during the sample extractions (draw-down) and the subsequent return to initial conditions (build-up) enables a value for formation permeability to be derived both for the draw-down and build-up phases of operation.
Another technique is described in U.S. Pat. No. 4,890,487, issued on Jan. 2, 1990, to Dussan et al. In this patent, a technique of measuring horizontal and/or vertical permeability is described. The pressure is measured while the fluid samples are extracted from a subsurface earth formation using a borehole logging tool having a single extraction probe. The pressure and flow data are analyzed to derive separate values for both horizontal and vertical formation permeability. The measured pressure profile is compared with its dimensionless pressure profile (obtained from known values of vertical and horizontal permeabilities).
Another technique that has obtained some widespread acceptance is a technique known as "Vertical Pulse Testing". In this technique, a packer is located along the production tubing to seal an area within the formation. A perforation is made on one location on the casing above the packer and in another location below the packer. The top (or bottom) perforated internal is produced while measuring pressures at the bottom (or top) perforated interval. The pressure drop is somewhat indicative of vertical permeability. However, to use this "Vertical Pulse Testing" method, computations must be made to solve two unknown parameters (vertical permeability and horizontal permeability). Flaws in the casing can cause flow behind the outer skin of the casing so as to affect values. In general, the technique of Vertical Pulse Testing has not proven as a reliable measurement of vertical permeability.
It is an object of the present invention to provide a method for the measurement of vertical permeability that provides an accurate assessment of the vertical permeability of a subsurface earth formation.
It is another object of the present invention to provide a method for the measurement of vertical permeability that can be used during the process of well formation.
It is a further object of the present invention to provide a method for the measurement of vertical permeability that requires no specialized equipment at the well site.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.